1. Field of the invention
The present invention relates to a novel method of producing a 2,4'-dihydroxydiphenylsulfone. More particularly, it relates to a method of producing a 2,4'-dihydroxydiphenylsulfone useful as developer for thermal recording paper efficiently and with high selectivity from phenol or a mixture of phenols and sulfuric acid.
The present invention also relates to a novel method of producing a high purity 2,4'-dihydroxydiphenylsulfone. More particularly, it relates to a method of producing a high purity 2,4'-dihydroxydiphenylsulfone useful as developer for thermal recording paper efficiently by purifying a crude 2,4'-dihydroxydiphenylsulfone produced from phenol or a mixture of phenols and sulfuric acid.
2. Description of the Prior Art
Various kinds of phenol compounds have heretofore been proposed as developer for thermal recording paper. Among the phenol compounds, 2,4'-dihydroxydiphenylsulfones have been expected to provide very useful developer for thermal recording paper.
Method: 4,4'-dihydroxydiphenylsulfones have heretofore been studied extensively and much efforts have been made to obtain high purity 4,4'-dihydroxydiphenylsulfones by minimizing formation of 2,4'-dihydroxydiphenylsulfones which are isomers thereof. However, no method of producing 2,4'-dihydroxydiphenylsulfone in increased yield has heretofore been reported.
When 2,4'-dihydroxydiphenylsulfones are produced in a conventional manner by the reaction of phenols with sulfuric acid, high purity products useful as developer for thermal recording paper have not been obtained. For example, when 2,4'-dihydroxydiplenylsulfone is produced by the reaction of phenol and sulfuric acid, a mixture of isomers containing 2,4'-dihydroxydiphenylsulfone and 4,4'-dihydroxydiphenylsulfone in a weight ratio of about 20 to 80 is obtained even under optimum reaction conditions. When 5-methyl-2,4'-dihydroxydiphenylsulfone is produced by using phenol and p-cresol as the material phenols, or when 3,5-dimethyl-2,4'-dihydroxydiphenylsulfone is produced by using phenol and 2,4'-xylenol as the starting phenol, a product containing at most 10 to 20 weight % of the 2,4'-dihydroxy isomer is obtained in either case.
Purifying 4,4'-dihydroxydiphenylsulfones has heretofore been studied extensively and much effort has been made to obtain high purity 4,4'-dihydroxydiphenylsulfones by minimizing the content of 2,4'-dihydroxydiphenylsulfones which are isomers thereof. However, no method of purifying 2,4'-dihydroxydiphenylsulfones with increased purity has heretofore been reported.